The Worst Rock in the World?Maybe. I'm sure there are plenty of candidates and personal "favorites."
Breccia is rock composed of other rock fragments that have been cemented together. Though it sometimes is stable, breccia is often hideously loose even though it is beautiful to look at, as it often forms spectacular cliffs and spires. The rock often looks like a knob-studded conglomerate seemingly made for climbing, but those knobs pull out easily and frequently, making climbing on them treacherous. Some hardened Wyoming mountaineers refer to the substance as "kitty litter." The name fits, as anyone who has climbed on it can surely attest.
Breccia can be igneous, sedimentary, tectonic, hydrothermal, or even impact-related (think meteorites) in origin. The breccia I have encountered in Wyoming is volcanic in origin, a hardened ash.
Breccia is abundant in the Southern Absarokas of Wyoming, and this page's gallery collects many breccia images from that range, though I have found a few from other areas and attached them. However, please feel free to add any breccia pictures you have from other mountain ranges, as Northwestern Wyoming is hardly the only place breccia is found.